High octane gasoline is required for modern gasoline engines. Previously, it was common to achieve octane number improvement by the use of various lead-containing additives. As lead has been phased out of gasoline for environmental reasons, it has become increasingly necessary to rearrange the structure of the hydrocarbons used in gasoline blending in order achieve higher octane ratings. Catalytic reforming and catalytic isomerization are two widely used processes for this upgrading.
The traditional gasoline blending pool normally includes C4 and heavier hydrocarbons having boiling points of less than 205° C. (400° F.) at atmospheric pressure. This range of hydrocarbons includes C4-C6 paraffins, and especially the C5 and C6 normal paraffins which have relatively low octane numbers. The C4-C6 hydrocarbons have the greatest susceptibility to octane improvement by lead addition and were formerly upgraded in this manner. With the phase out of lead additives, octane improvement was obtained by using isomerization to rearrange the structure of the paraffinic hydrocarbons into branched-chain paraffins or reforming to convert the C6 and heavier hydrocarbons to aromatic compounds. Normal C5 hydrocarbons are not readily converted into aromatics; therefore, the common practice has been to isomerize these lighter hydrocarbons into corresponding branched-chain isoparaffins. Although the C6 and heavier hydrocarbons can be upgraded into aromatics through dehydrocyclization, the conversion of C6 hydrocarbons to aromatics creates higher density species and increases gas yields with both effects leading to a reduction in liquid volume yields. Moreover, the health concerns related to benzene have lead to restrictions on benzene and aromatics. Therefore, it is preferred to change the C6 paraffins to an isomerization unit to obtain C6 isoparaffin hydrocarbons. Consequently, octane upgrading commonly uses isomerization to convert C6 and lower boiling hydrocarbons.
The Reid vapor pressure (RVP) of gasoline has been utilized by the Environmental Protection Agency as a means of regulating volatile organic compounds emissions by transportation fuels and for controlling the formation of ground level ozone. As these regulations become more stringent and as more ethanol (which has a high vapor pressure) is blended into gasoline, C5 paraffins need to be removed from the gasoline pool. Moreover, the need to remove components may also extend to some C6 paraffins. This may result in refiners being oversupplied with C5 paraffins and possibly C6 paraffins, forcing them to sell these products at prices lower than gasoline blendstock.
Commercial refiners face a number of problems utilizing hydrocarbons with molecular weights less than about 90 in gasoline. Some refiners are limited in the amount of light naphtha, particularly pentanes, they can add to gasoline in the summer months because of more stringent regulations on vapor pressure. In addition, some refiners cannot blend all of the high octane reformate they produce into gasoline because of new regulations limiting the total aromatics to 35 vol %. The growth of shale crude oil production has increased the amount of low value butanes and pentanes produced in refineries. Finally, some refiners need to purchase isobutane as feedstock for C3 and C4 olefin conversion to gasoline in existing alkylation units.
Therefore, there is a need for processes which allow better utilization of light and heavy naphtha in refineries.